Support private repositories

[La0]

Sample TC payload:

        payload:
          maxRunTime: 3600
          image: python:${pyver}-slim
          features:
            taskclusterProxy: true
          command:
            - /bin/bash
            - '--login'
            - '-x'
            - '-c'
            - >-
              echo "[global]\ndisable-pip-version-check = true\nno-cache-dir = false\n" > /etc/pip.conf &&
              apt-get update -qq &&
              apt-get install -y -qq --no-install-recommends --no-install-suggests build-essential curl git openssh-client &&
              cd ~ &&
              mkdir .ssh &&
              ssh-keyscan github.com > .ssh/known_hosts &&
              curl -L ${secret_url} | python -c 'import json, sys; a = json.load(sys.stdin); open(".ssh/id_ed25519", "w").write(a["secret"]["key"])' &&
              chmod 0400 .ssh/id_ed25519 &&
              git clone --quiet ${ssh_repo} repo &&
              cd repo &&
              git -c advice.detachedHead=false checkout ${head_rev} &&
              pip -q install tox &&
              tox

[La0]

Not needed anymore for fuzzing.